This invention relates, in general, to a method for trimming an electrical component that is located within an integrated circuit and, more particularly, to a method for trimming implanted or diffused resistors by the use of metal migration.
The term "trimming" is used to mean the fine adjustment of resistance, capacitance, or inductance in a circuit. As used herein, "metal migration" refers to the movement of metal into a semiconductor crystal (e.g. a resistor). This movement of metal is caused by sending current pulses through the resistor. These pulses are usually of high amplitude and small pulse width.
In the manufacture and packaging of integrated circuits, it is sometimes necessary to "match" (make electrically identical) resistors, transistors, diodes or to set the absolute value of a single electronic device to a certain value, such as trimming a resistor to set a particular current level. For example, trimming of component values on an integrated circuit chip can result in minimizing the input offset voltage of an operational amplifier, or its temperature coefficient of input offset voltage. Trimming of a voltage reference can set the output voltage and temperature coefficient to precise values. Trimming of D/A or A/D converters is commonly done in order to obtain increased accuracy.
Resistor trimming is by far the most common means of adjusting electrical parameters of integrated circuits. Two methods are generally used. The first is to use a trim potentiometer outside the semiconductor device but connected to it through external pins, i.e. on a printed circuit board. The second method is to trim a resistor on the integrated circuit die itself.
In the prior art, electronic components, e.g. resistors, have been trimmed by mechanical, electrical, or chemical means. For example, mechanical means include abrasion, usually by sandblasting, and laser shaping of a conductive layer. Electrical means include fuse blowing, i.e. vaporization of metal by passing high current therethrough and short-circuiting diodes with excessive current. The latter is also known as "zener zapping" since the diodes connected along the resistive elements are zener diodes. Chemically changing the conductivity of the resistor by anodizing the metal is possible but not often used in a production environment.
The known methods of resistor trimming on the die have serious disadvantages. The cost of laser trimming equipment is high. The maintenance and programming of such equipment are extremely critical and costly. Fuse blowing can cause surface contamination and cracks in the protective glass layer which, in turn, cause reliability problems. Zener zapping can require that over twenty per cent of the die be dedicated to resistor trimming.
A further disadvantage in the known methods of resistor trimming on the die is that after the die is trimmed (usually in wafer form) it is scribed and assembled in a package. This assembly process can put stress on the semiconductor chip which can cause the trimmed resistor to change due to piezoresistive effects. It would, therefore, be advantageous to be able to trim an on chip resistor after the integrated circuit die is in the package.